The present invention relates to an apparatus for storing and reproducing digital signals of the helical scan type employing a rotary head. More particularly, the invention relates to an apparatus for storing and reproducing digital signals employing a rotary head of small diameter and compact size, but in which the same tape format as used previously is retained.
One such apparatus is a rotary head type digital audio tape recorder (R-DAT) recently placed on the market. An R-DAT includes two heads, mounted on a rotary drum and having different azimuths. With an R-DAT as shown in FIG. 1, a track length L, a track angle .THETA., and a track pitch P determine the tape format on a magnetic tape T that is running at a predetermined speed v. The rotational speed R per unit time of the rotary drum, the drum diameter D, and tape wrap-around angle .alpha. may be set at will as far as the requirements for the track length, track angle and track pitch are met.
Early R-DATs commonly employed certain recommended values for the above requirements, i.e., a rotational speed R of 2000rpm, a drum diameter D of 30 mm, and a tape wrap-around angle .alpha.of 90.degree.. Recently, it has been proposed that a drum diameter D of 15 mm be employed to obtain a more compact apparatus, together with a wrap-around angle of 180.degree. and a rotational speed of 2000 rpm.
FIG. 2 shows the general circuit arrangement of an apparatus as proposed above. The apparatus include a mechanical unit 1, a high frequency (RF) unit 2, an ATF (Automatic Track Finding) processing unit 3, a PLL (Phase-Locked Loop) 4, a servo unit 5, a signal processing unit 6, an audio signal unit 7, and a system control unit 8.
In the aforementioned apparatus, when storing data, an analog signal applied to an analog signal input terminal 9 is filtered to remove unwanted high frequency components thereof by a low pass filter (LPF) 71 in the audio signal unit 7. The output of the LPF 71 is supplied to an A/D converter 72 to be converted into a digital signal. The digital signal from the audio signal unit 7 is input to a PCM subcode encoding circuit 61 in the signal processing unit 6. The PCM subcode encoding circuit 61 effects interleaving of these data stored in the RAM 65 and produces and adds correction codes and subcodes. Thereafter, these data are again stored in the RAM 65 under control of a RAM control unit 64. The output of the RAM 65 is converted by 8/10 converter 62 then input to a recording amplifier 21 in the RF unit 2. In the RF unit 2, a switch 23 is set to a "REC"position when an REC control signal from the system control unit 8 is at the "H" level, and thus the data from the recording amplifier 21 is supplied through the switch 23 to heads 11a and 11b so that the data is recorded on the magnetic tape T. The heads 11a and 11b are provided on a drum 11 to perform data recording on and reproduction from the tape T.
For reproduction, the signals that are read through the heads 11a and 11b are input to a reproduction amplifier 22 in the RF unit 2 through the switch 23. The output of the amplifier 22 is input to a PLL circuit 41 in the PLL unit 4 where period detection is effected to separate the data into playback data (PBDATA) and a playback clock (PBCK). The PBDATA and the PBCK signals are input to an 8/10 demodulator 67 in the signal processing unit for demodulation, and the resulting output is applied to the RAM 65 to be stored in order under the control of the RAM control unit 64.
Connected to the RAM 65 is a PCM subcode decoding circuit 66 which performs error correction and deinterleaving of the data stored in the RAM 65. The output of the circuit 66 is input to a D/A converter 73 in the audio signal unit 7 to be converted into an analog signal, which in turn is supplied to an LPF 74 for removal of the unwanted high frequency components thereof. Thereafter, the analog signal is output from an analog signal output terminal 10. Within the signal processing unit 6 is provided a fixed frequency generator 63 for generating various fixed frequency signals such as ATF. The fixed frequency signals provided by the generator 63 are recorded at a predetermined location on the magnetic tape by shifting the switch 68 at a predetermined timing.
A rotary drum 11 of the mechanical unit 1, a capstan 12, a supply reel 13, and a take-up reel 14 are controlled through servo control. For this purpose, a frequency generator (FG) 11cand a pulse generator (PG) 11d are provided for the rotary drum, a frequency generator FG12a for the capstan 12, and frequency generators FG13a and FG14a for reels 13 and 14, respectively. The outputs from the FG 11C and the PG 11d are input to a drum servo circuit 51 in the servo unit 5, while a reference signal from a reference signal generator 54 and the clock PBCK from the PLL unit 4 are input to the drum servo circuit 51, thereby applying phase servo control to the drum motor (not shown) so that reproduction and recording of data may be effected when the heads 11a and 111b are in contact with the tape T. The output from the FG 12a is input to a capstan servo circuit 52 of the servo unit 5 together with a reference signal from the reference signal generator 54 and an error signal indicative of an amount of off-track error from the ATF processing unit 3 to thereby servo-control a capstan motor (not shown) for driving the capstan 12. The outputs of the FG 13a and 14a are input to the reel servo unit 53 in the servo unit 5 together with the reference signal from the reference signal generator 54 to servo-control the reel motor (not shown) for driving the reels 13 and 14.
In the example shown, the drum diameter D is 15 mm and the tape wrap-around angle .alpha. is 180 degrees. The reference signal is input from the reference signal generator 54 to the drum servo circuit 51 so that the drum 11 rotates at 2000 rpm for both recording and reproduction modes when the apparatus is in the SP mode, and rotates at 1000 rpm for both recording and reproduction modes when the apparatus is in the LP mode. Also, the capstan servo circuit 52 is supplied with a reference signal which sets the tape speed v to 8.15 mm/sec in the SP mode and to 4.075mm/sec in the LP mode.
For reproduction, the amount of off-track error is detected from the reproduced signal in both the SP mode and the LP mode for controlling the advancement of the tape on the basis of the amount of off-track error to thus accurately trace the track on which a signal has been recorded. The automatic control of the track trace when reproducing the signal stored is called ATF (Automatic Track Finding). For ATF, the reproduced signal is input to the digital unit 31 in the ATF processing unit 3 to detect a sync signal from the reproduced signal, thereby outputting sample pulses SP1 and SP2 for sampling the amounts of off-track error of adjacent tracks on both sides of a presently traced track. These pulses are input to an analog unit 32 of the ATF processing unit 3 to detect the difference between the amounts of off-track error of the two adjacent tracks, the difference signal being employed as a tracking control signal for reproducing operations.
A reel servo circuit 53 mainly functions in such a way that the square of the sum of the FG periods of the FG 13a of the supply reel 13 and the FG 14a of the take-up reel 14 in the mechanical unit 1 is maintained constant during search operations, thereby maintaining a constant speed of tape advancement. A system controller 81 in the system control unit 8 controls the writing and reading operations of subcode signals, while a mechanical controller 82 controls the mechanical unit 1.
The operation of the aforementioned arrangement will now be described in more detail with reference to the timing chart of FIG. 3.
In the SP mode, the rotary drum 11 rotates at 2000 rpm for both writing and reading, and thus the pulse generator 11doutputs a switch signal SWP having a period of 30 msec, as shown in FIG. 3 at (a), in accordance with the outputs of the heads 11aand 11b.
In the write mode, an REC signal is at the "H" level, as shown in FIG. 3 at (d). PCM data is then recorded in two tracks, i.e., a track A and a track B, which together form a frame. Both tracks are assigned the same frame address. The PCM data is required to be processed and stored within 30 msec. The recording data (REC DATA) can be recorded at any time within this period, as shown in FIG. 3 at (c). The digital data from the A/D converter 72 is written into the RAM 65 during a time period of 30 msec between A.sub.0 and B.sub.0 of the switch signal SWP as shown in FIG. 33 at (b), is then interleaved (C1 and C2), and is read out of the RAM 65 during a time period between A.sub.1 and B.sub.1 equivalent to a unit rotation of the rotary drum.
Into the REC DATA, as shown in FIG. 3 at (c), is inserted signals of fixed frequencies required for the established DAT format, such as an ATF sync signal and pilot signal, at a predetermined timing by shifting the switch 68.
On the other hand, the REC signal is at "L" , as shown in FIG. 3 at (e), and the reproduced high frequency signal (PBRF) is a continuous waveform, as shown in FIG. at (f). A reproduced clock (PBCK) is sampled from this PBRF in the PLL unit 4, as shown in FIG. 3 at (g). Also, as shown in FIG. 3 at (h), only subcodes and PCM portions are written as reproduced data (PB DATA) into the RAM 65.
Reading of D/A DATA from the RAM 65 for D/A conversion by the S/A converter 73 is sequentially carried out with the same frame address every 30 msec, as shown in FIG. 11 at (i). At this time, tracking is effected by sampling the amount of off-track error of the two adjacent tracks on the basis of the sampling pulses SP1 and SP2 shown in FIG. 11 at (j) and (h), respectively, generated by the digital unit 31 in the ATF processing unit 3, which detects the ATF sync pattern for generating the sampling pulses.
As far as the apparatus operates under the above described conditions, i.e., a drum diameter of 15 mm, a wrap-around angle of 180 degrees, a rotational speed of 2000 rpm in the SP mode and 1000 rpm in the LP mode, the rotary drum 11 rotates with a certain period t, the tape T runs at a predetermined speed, and the heads A and B trace the tracks A and B, respectively, as shown in FIG. 4 (for reproduction), no problems occur.
However, with the above described prior art arrangement, not only the tape speed v but also the drum rotation in the LP mode must be half those in the SP mode. Thus, if the drum diameter D is 30 mm, the RF signals obtained in the SP and the LP modes will be as shown at (a) and (b), respectively, in FIG. 5, while, if the drum diameter is 15 mm to allow for a more compact size of the apparatus, the RF signals obtained in the SP and the LP modes will be as shown at (a) and (b), respectively, in FIG. 6.
As is apparent from comparing the two figures, reducing the drum diameter D to 15 mm decreases the speed of the tape relative to the head in the LP mode to one-quarter of the recommended value in the SP mode, thereby causing problems such as a high error rate due to the decreased signal level.
Increasing the drum rotational speed to, for example, 4000rpm in the SP mode and 2000 rpm in the LP mode to increase the relative speed between the tape and the head will overcome such problems but causes another problem in that the respective heads cannot accurately trace the corresponding tracks.
An R-DAT is capable of recording data at a high data density and long recording times are possible. These advantageous properties though are accompanied by difficulties in searching for a desired tune on a tape.
The R-DAT is provided with a high speed search function in which the tape is made to run at a speed about 200 times greater speed than that used in reproduction in the SP mode for reading an ID (S-ID), a program number (PNO), and a time code, i.e., information indicative of the start of a tune which has been recorded in subcode areas on both sides of the PCM area on the tape. In order to read this information during a high speed search, it is necessary to set the relative speed between the tape and the head equal to that during reproduction. Because a change in the relative speed causes a change in the frequency range of the read-out signal, the frequency responses of the head, the amplifier, and equalizer must be changed and the frequency range of the PLL unit and the signal processing unit must be widened.
The dotted lines a and b in Fib. 15 show the relationship between the search speed and the drum rotational speed. The dotted line a indicates the relative speed corresponding to a drum rotational speed of 2000 rpm. As is apparent from the graphs, when searching at a tape speed 200 times faster than that used for normal reproduction, the rotary drum 11 must rotate at about 6000 rpm, nearly 1.5 times faster than for reproducing.
The rotary drum 11 is driven by a drum motor, the rotational speed of which can be varied by varying the voltage applied thereto. Thus, varying the rotational speed over a wide range requires varying the voltage applied to the drum motor over a wide range. Consequently, provision of an adequate response in the servo channel requires a voltage source of high voltage, which is a serious problem with a portable R-DAT operating on a battery source.